A process as described above is known from No. EP-C-4122. This process is suitable for industrial-scale application. The highest efficiencies according to No. EP-C-4122 are achieved at low temperatures, for instance up to 20.degree. C. On a technical scale cooling to such a relatively low temperature of such a strongly exothermic reaction necessitates costly investments as ammonia or nitrite cooling, for instance, is to be applied. If the reaction is carried out at a higher temperature, for instance at 40.degree. C., a simpler cooling system, such as cooling water will suffice, it is true, but then the selectivity of the hydrogenation is found to become substantially lower.